The number of frames per second (abbreviated to FPS) technically describes the rate of video frame update of the whole screen per second, but mostly can impact image transition smoothness. This rate is often described in hertz (Hz), a frequency in cycles per second. Generally, the frames per second of a game does not help if it exceeds the refresh rate of the monitor (i.e. 60 Hz = 60 frames per second refresh on the monitor).
In games like World of Warcraft, a lot of pressure is put on trying to keep the rate as high as possible, but is generally considered more important in faster action games like first-person shooters.